1. Field of the Invention
This invention relates to a camera that is capable of carrying out flash photography using a strobe, and particularly to controlling an exposure during pre-flash photography.
2. Description of the Related Art
A recent digital camera with a flash photography/shooting function carries out pre-flash photography before main flash photography to accurately detect the distance of an object. When a release button is depressed fully, strobe light is emitted from an electronic flash and a distance from the camera to a target object is detected using light reflected off the target object. Then, an amount of light (Guide Number) to be emitted from an electronic flash/strobe during main flash photography is determined based on the object distance detected in the pre-flash photography, and the main flash photography is carried out on the basis of the obtained Guide Number.
In the case of a compact type digital camera, pre-flash photography utilizing a “through-image” mode is carried out. Concretely, when a release button is depressed fully, a through-image, which is a real-time image captured by a lens, is displayed on a monitor, the image-pixel signals are read immediately after the button is fully depressed and strobe light is emitted from the electronic flash. Then, an object distance is calculated from the difference between image-pixel signals generated in pre-flash photography and image-pixel signals generated before the pre-flash photography. Thus, an amount of light, i.e., Guide Number appropriate for an object is determined. The pre-flash photography process is disclosed, for example, in JP2007-86181A1 and JP2002-359774A1.
The above pre-flash photography is carried out regardless of the ambient illumination of the photography location. An exposure value for pre-flash photography is not selected by a user or an auto exposure operation, but determined in advance from an expected ambient illumination and/or expected object distance. Therefore, pre-flash photography with improper exposure is carried out occasionally. When an object to be photographed is very close to a camera, an amount of light that bounces off the object may exceed a range that is appropriate for exposure, in which condition image-pixel signals read from an image sensor include saturated signals. Also, when an object is relatively far from the camera and located in a position that can be reached by only a limited amount of strobe light, output levels of the read image-pixel signals are extremely low because of an insufficient amount of reflected light, so that an accurate object distance cannot be detected.